As used herein, the term "bearing loads" refers to loads in a panel or other structure substantially parallel to the plane (flat or curved) of the panel or other structure. In the case of loads around a fastener hole, "bearing loads" refers to loads essentially perpendicular to the axis of the hole and/or the shank of a fastener received in the hole.
It is known to fabricate structural panels from a composite material comprising layers of reinforcing fibers (e.g. graphite fibers) within a resin matrix. The use of such composite materials for making structural and nonstructural aircraft parts is well-known and widely recognized for its benefit of providing high strength with low weight.
Damage to an aircraft, such as damage caused by projectiles, can result in holes in composite material panels. In addition, fastener holes may be mislocated or oversized due to misdrilling or wear. A standard way of repairing, relocating, or resizing such holes, known as the "scarf" method, is to form a relatively wide, tapered recess in the composite material around the hole. Disk-shaped layers of fiber reinforced/resin matrix composite material, of different diameters, are set into the recess to form a patch. The fibers of each layer are generally oriented at a different angle than the fibers of adjacent layers for strength. The layers are secured in place by the use of a high shear strength adhesive.
The standard "scarf" method has a number of drawbacks. Practicing the method requires a highly trained technician. In order for the patch to properly carry bearing loads, especially around fastener holes, the fibers in each layer must be oriented in a bearing load plane. This orientation is very difficult to accomplish. The adhesives used require a long cure time, usually in combination with applied heat and pressure. In addition, the method converts the original bearing load to an adhesive shear load since the repaired area relies almost totally on the adhesive to transfer the design load across the repair. This is due to the wide angle of taper and the resulting orientation of the interface between the patch and the parent structure, which deviates substantially from an orientation perpendicular to the bearing load. When a fastener hole must be formed in the repaired area, the orientation of the patch fibers is particularly critical in order to assure the load-bearing integrity of the repaired panel. Moreover, the wide taper angle may cause other fastener holes to be within the radius of the scarf patch. This is especially likely when the panel or other structure is relatively thick. The load bearing integrity around these other holes may also be affected, and the other holes will generally require redrilling.